1. Field of the Invention
The present invention relates to a semiconductor device. More specifically, the present invention relates to a film for protecting a semiconductor device formed to prevent disconnection of a conductive layer formed on a phosphorous silicate glass layer in which the phosphorous is highly doped.
2. Description of the Prior Art
One important factor determining reliability of electronic components is the air tightness of the package. In order to attain high degree of air tightness, a hermetic seal such as a metal package, a ceramic package, a glass seal and the like has been used. On the other hand, a resin mold is also often used in consideration of a easier workability and a better cost advantage in spite of the decreased air tightness. Problems involved in a resin mold are that the air tightness of the resin itself is inferior to that of metal, ceramic and the like; the adhesive property of the resin to metal is inferior to that of glass to metal, and many impurities are included in a resin material and the like. As countermeasures therefore, passivation of the surface has been developed with respect to devices, and approaches such as a buffer coat process embedding of a moisture permeable agent in a resin and the like have been employed with respect to a package. In addition, a material of a smaller hygroscopic permeability of moisture and a better adhesive property to metal has been selected with respect to resin.
Generally, the electrical characteristic of a semiconductor device such as a transistor, a large scale integration, or the like is sensitive to atmosphere, such as moisture, detrimental impurities and the like. One major cause of failure is disconnection due to corrosion of an aluminum electrode layer generally used in a semiconductor device. In particular, in sealing a semiconductor device with a resin of epoxy group, it is necessary to take into full consideration protection from moisture in view of the fact that the resin provides poor protection against moisture. Necessity of such protection is well-known to those skilled in the art from many various papers presented in the past. The present invention is directed to an improvement in such protection or in passivation of the surface of a semiconductor device. A conventional method employed in semiconductor publication was to cover with a film of silicon oxide (SiO.sub.2) or a film of silicon nitride (Si.sub.3 N.sub.4) on an aluminum electrode layer formed on a semiconductor device. Silicon oxide (SiO.sub.2) and silicon nitride (Si.sub.3 N.sub.4) films have similar performances with respect to cracks, dirt and the like; however, a film of silicon nitride (Si.sub.3 N.sub.4) provide better protection with respect to contamination from moisture, as compared with silicon oxide (SiO.sub.2). Nevertheless, it was not able to fully prevent an influence of moisture in a semiconductor device with a phosphor silicate glass layer even by using these approaches.
The inventors of the present invention investigated the cause of the above described problems and as a result observed the facts set forth in the following. Now such cause will be described in detail with reference to one example of a conventional semiconductor device shown in FIG. 1.
FIG. 1 is a sectional view of a portion of a conventional N channel silicon gate MOS integrated circuit using a phosphorous silicate glass layer. A semiconductor device 1 comprises a silicon substrate having P type conductivity, upon which a thermally grown thick oxide film or a field oxide film 2 is formed by a well-known LOCOS (Local Oxidation of Silicon) method, a thermally grown thin oxide film 5 is formed on the portion where a drain regin 3 and a source region 4 are to be formed, and a thermally grown gate oxide film is formed on the portion where a gate portion is to be formed, whereupon a first electrode including an electrode layer 6 and a gate electrode 7 is formed by polycrystalline silicon on the silicon oxide films 2 and 5, respectively. A diffused layer, i.e., the drain region 3 and the source region 4 are formed by duffusing an N type impurity such as phosphorous and at the same time a thin film 8 of silicon dioxide is formed on the surface of the gate electrode 7, the electrode layer 6, the drain region 3 and the source region 4. A phosphorous silicate glass layer 9 is formed on the silicon oxide films 2 and 8. The thin silicon oxide film 8 and the phosphor silicate glass layer 9 on the part of the drain region 3 and the source region 4 are removed by an etching process and, then a second electrode, i.e., an aluminum electrode 10 for connecting elements is formed on the phosphor silicate glass 9 so as to be in ohmic contact with the drain region 3 and the source region 4. The phosphor silicate glass 9 is formed for the purpose of preventing instability due to mobile ions such as alkali ions and for the purpose of preventing disconnection of the aluminum electrode 10. A silicon nitride film 11 is formed on the phosphor silicate glass 9 and aluminum electrode 10 for the purpose of protection, because the phosphor silicate glass 9 has a hygroscopic property.
On the other hand, although a thick oxide film is not formed by the LOCOS method on the scribing line 12 for dividing the semiconductor substrate 1 of a wafer form into chips, the phosphor silicate glass layer 9 and the silicon nitride film 11 are formed after the step of removing the oxide film formed on the occasion of formation of the drain and source regions 3 and 4. However, the above described phosphor silicate glass layer 9 and the silicon nitride film 11 are removed by an etiching process using the same mask before the scribing process. Accordingly, the surface of the phosphorous silicate glass layer 9 is covered and protected with the silicon nitride film 11 but the end surface of the phosphorous silicate glass layer 9 is exposed at the end thereof, which allows moisture to be absorbed from the end thereof and accordingly allows the internal aluminum electrode 10 to be corroded.
FIG. 2 is a sectional view showing the end portion of the semiconductor device of the above described structure. The phosphorous silicate glass 9 formed on the silicon oxide film 2 absorbs moisture from the exposed portion at the end to be softened and the aluminum electrode 10 comes to be corroded when the moisture reaches the aluminum electrode 10, whereby the silicon nitride film 11 and the aluminum electrode 10 comes to be peeled off from the semiconductor substrate 1, with the result that the semiconductor device becomes inoperable eventually.